There are many examples of vessels wherein the contents are subjected to heating or boiling. Examples are autoclaves, distillation vessels, boilers, etc. With the exception of vessels such as pressure cookers, wherein pressure build-up is intentional and the vessel is constructed to withstand internal pressures, it is customary to provide an outlet for vapour generated during the boiling or distillation process. In such cases, it is also desirable to provide a pressure relief means which will allow vapour to be vented from the vessel in the event that the internal pressure rises to a dangerous level, caused by blockage of the vapour outlet. An example of a situation where such a blockage could occur is where the vessel has an internal liner, as may be the case with a distillation vessel. Such a liner may be in the form of a bag inside the distillation vessel, as described in Canadian Patent Application No. 2,277,449. The material to be processed—for example, used solvent from a cleaning process—is placed in the bag and the vessel heated to drive off the solvent vapour, which exits through an outlet to a condensation/collection vessel. In the event the outlet opening would ever be blocked, for safety reasons there should be some kind of anti-pressure device incorporated into the distillation vessel, such as an anti-pressure valve placed on the cover of the vessel to release before any significant pressure build up can occur. In the aforementioned Canadian Patent Application No. 2,277,449 the cover itself was designed to release at almost any sign of positive pressure. However during field tests it was found that, in certain instances, this anti-pressure device was not effective against a dangerous pressure build up and release. The cause was the bag being overfilled with solvent, which overflowed behind the bag. As the vessel heated up, the solvent behind or outside the bag began to vaporize and expand. As the vapour expanded, more liquid from inside the bag overfilled to the outside of the bag, creating more vapour until the bag was pushed upwardly from underneath and eventually covered the vapour exit. Now, with the vapour exit blocked, the cover should have released, but did not, as the bag being pushed from the inside out in a balloon effect pushing on the cover and eventually pushing the cover open could not release the pressure, because the bag itself was sealing the opening. The bag continued to expand inside the vessel until finally hot solvent burst from the inside to the surrounding area. This, of course, is very dangerous and could occur in any container that has the possibility of its normal vapour outlet and any pressure release device being blocked.
Hence there is a need for a more reliable method of protecting a normally un-pressurized vessel from dangerous pressure build up, resulting from its vapour outlet being blocked, regardless of what other pressure release devices (if any) may be present on the vessel.